1. Field of the Invention
The present invention relates to a novel denatured bismaleimide (hereinafter "BMI") resin shown by the following formula (I) and to a process for preparing same. More particularly, the present invention relates to the denatured bismaleimide oligomer of repeating units represented by the following formula (I) which comprises the Michael addition reaction of a bismaleimide and an aromatic secondary diamine: ##STR2## wherein, n represents an integer of 1 to 3,
A represents a divalent group having from 2 to 20 carbon atoms,
A' represents a divalent group having less than 20 carbon atoms which includes at least one or more aromatic groups, and
R represents aliphatic group having from 1 to 10 carbon atoms.
The BMI resin of the present invention has excellent processability and improved toughness without a decrease in heat resistance. Especially, the prior problem, that is, the problem of the viscosity of the resin increasing suddenly as crosslinking progresses, can be solved and the pot life of the resin (namely, the maximum usable time of the resin which is not cured yet in spite of addition of a curing agent) is improved to a considerable degree. Therefore, the BMI resin of the present invention can be cast into a complicated shape. And, in case of the composite materials the hot-melt process can be carried out without having to use any solvent, as well as not having to use a polar solvent having a high boiling point. When the prepreg for production of composite materials is prepared using the BMI resin by means of a hot-melt process, there is an advantage in properties of the materials formed, because no solvent, such as a polar solvent having a high boiling point, had been used in the formation thereof.
2. Description of the Prior Art
Generally, reactive oligomers are low molecular weight materials which contain a moiety showing the property of a completely cured resin. They have been used as structural materials for improving processability of prior heat resistant resins having excellent heat stability.
Bismaleimide resins may undergo shaping in the state of oligomers. Their thermal and mechanical properties, when cured, and the shaping conditions thereof, may be greatly influenced by frame structures and molecular weights thereof.
Recently, bismaleimide resins have been widely used as matrix resins of the most recent composite materials in various fields of updated technologies, such as the aeronautical space industry and the defense industry, as well as in the heat resistance-required fields such as the electronics industry, the auto industry and so on. This is due to the fact that they have excellent heat-oxidation stability, hot-wet strength, dimension stability, flame retardance and processability. Also these materials are relatively inexpensive.
The resins produced by polymerization of only the bismaleimides alone, have generally exhibited distinct heat resistance of 300.degree. C. or more of the second transition temperature (Tg) because of their high crosslinking densities. On the other hand, these resins have some disadvantages, namely, they are brittle and soluble in only polar solvents having a high boiling point, such as N-methyl-2-pyrrolidone (b.p. 202.degree. C.), N,N',-dimethyl formamide (b.p. 153.degree. C.), and the like. In the case of producing a prepreg by means of impregnating carbon fiber with a polar solvent, the produced prepreg contains a large amount of residual solvent because it is difficult to remove the polar solvent. Also, in the case of laminating the prepreg, the residual solvent acts as a plasticizer and it volatilizes at a high temperature. As a result, the properties of the prepreg are reduced to an undesirable level.
A representative bismaleimide resin which is on the market, is Kerimid 601 (manufactured by Rhone Poulenc Co. in France). It has a molecular weight of about 1,000, which is reactive oligomer terminal-treated with maleimide, and it is prepared by the Michael addition of N,N'-4,4'-diphenylmethane bismaleimide and 4,4'-diaminodiphenylmethane as a primary diamine as shown in U.S. Pat. No. 3,562,223. In the case of preparing the bismaleimide resin in accordance with the method disclosed in U.S. Pat. No. 3,562,223, the molecular weight of the oligomer increases through chain extension, and consequently the crosslinking density, when cured, is reduced. Although the heat resistance deteriorates slightly, the resin retains a toughness to some extent. At this time, when the amount of primary diamine is too excessive, the resin tends to lose its value as a heat resistant resin because it is easily degraded at a high temperature.
According to the cited U.S. patent, the preferable mole ratio of bismaleimide to primary diamine has been found to be 2.5 to 1, at which point the commercialization of the bismaleimide resin becomes possible.
However, Kerimid 601 resin, the subject matter of U.S. Pat. No. 3,562,223, is prepared using N-methyl-2-pyrrolidone as a polar solvent having a high boiling point. Therefore, the resin has only limited use, for example, as the matrix resin of a multi-layer printed circuit board in the electronics industry, because of its high crosslinking density. According to the patent, after forming oligomers of low molecular weight by the Michael addition of a bismaleimide and a primary diamine at a low temperature, the oligomers are cured at a high temperature whereby, crosslinking occurs by homopolymerization of the terminal double bonds of the bismaleimide, and additional crosslinking by the diamine occurs simultaneously. As a result, the resin exhibits a very high crosslinking density.